Recording media, e.g. DVDs, on which copyrighted works such as movies and music are recorded in digital form, are widely used in recent years.
The DVD standard employs playback path control based on navigation data. According to this technique, a playback path of content can be specified using navigation data. Also, the playback path can be interactively altered by the user through a remote control (see Japanese Patent Application Publication No. H10-336568).
Meanwhile, a content delivery system that uses DRM (Digital Rights Management) is becoming common in recent years. According to DRM, a license is provided separately from encrypted content. The license contains a license key and a use condition. The encrypted content is decrypted using the license key and the decrypted content is played back, based on the use condition.
Consider a case where DRM is used in playback path control over a DVD on which a movie is recorded. A playback device selects content through playback path control, and checks a use condition of the content. Here, suppose the use of the content is not permitted under the use condition. If the playback device plays the content despite the use condition, the copyright of the content is violated. If the playback device does not play the content in compliance with the use condition, on the other hand, part of the scenes of the movie will end up being missing because a preset playback path is deviated. This causes ambiguity or contradiction in the story of the movie, thereby confusing the viewer.